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Petrol into Diesel; Diesel into Petrol


If you’ve ever put the wrong sort of fuel in your car, you are not alone.  When one member of my family (who will remain nameless*) filled a diesel-powered work ute with petrol the other afternoon, the local garage said that it was the second time it had happened that day.

What happens if you put the wrong sort of fuel in your car?  If you drive a diesel and have put petrol in the tank, you’re in real trouble.  Petrol acts as a solvent and reduces the amount of lubrication that diesel can do to the engine parts – and diesel engines need that lubrication.  The sooner that you realise that you’ve put in the wrong fuel, the better.  If you’re still at the bowser when you realise what a nitwit you’ve been, don’t start your engine.  The petrol can be drained from the fuel tank (along with everything else in the tank) and the wretched stuff won’t get into your engine and ruin it.  If you start the engine – well, there goes your engine!  They say that if petrol does get into the workings and start chugging around inside, a whole new motor is going to be cheaper than an attempt to repair it.  If the car is on the older side, a whole new car might be in order.

What about the reverse?  What if you put diesel into a petrol engine?  This is less of a disaster and you won’t kill your engine if you switch the ignition on.  What will happen when you switch the ignition on is… not a lot.  You see, diesel engines have a lot more compression than petrol engines to get the air hot enough to ignite the fuel.  Petrol engines just don’t have what it takes.  This is a very short description and there’s a fair bit of science going on here, but let’s stick to the point.  Again, you’ll have to get towed away so your car can have its tank pumped out but if a bit of diesel’s got into the system, it isn’t the end of the world.  But prepare for some rough running.

Manufacturers of cars and of bowsers have sensibly made the diesel and the petrol nozzles, and the holes they go into, different sizes.  Diesel needs a bigger nozzle, so the chances that you’ll shove diesel into a petrol is slim, unless you have an older vehicle.  This won’t help you if you try to put petrol into a diesel, as you can fit a small nozzle in a big hole.  However, the way the nozzle wobbles around should tip you off.

The expense, hassle and humiliation of having to get a full tank of gas pumped out of your vehicle and chucked away can be avoided by either (a) not going on autopilot when at the bowser if you have two cars with different fuels or (b) getting one of the petrol pump attendants to do it for you.  If they make a mistake, it’s their expense and hassle.  Besides, you get to talk to a real human being (great after a day behind a computer or making endless phone calls) and ensure that your local garage keeps employing teenagers who would otherwise be making trouble.

DieselFuel_195121818*No, this person was not me. I know about this event because I got the call and had to head out with my big Ford Fairlane to tow this person to the mechanics so their car could have its stomach pumped.  Family member is getting enough leg-pulls from everyone else so there’s no need to preserve their name for eternity online.


  1. Shane says:

    Good article. I feel the problem will get worse as diesel cars are now so quiet and fuel retailers show little care because at fuel outlets both petrol and diesel is dispensed from the same bowser set. The brains trust at Caltex even thought marketing ‘Vortex’ for petrol and diesel was helpful. I loved my diesel bus and troop carrier, but I stay from diesel cars as when you do the numbers, especially unreal service costs, all up it works out the same cost. Next there’s the time wasted as you wait for that one diesel bowser (newer places have two), same with LPG, you begin to hate buying fuel.

    November 18th, 2013 at 4:16 pm

  2. Dave says:

    Very true about the use of “Vortex” as the name for both premium petrol & diesel fuel Shane. A friend of mine got caught through exactly that act of sheer genius.

    November 25th, 2013 at 9:25 am

  3. Alex says:

    You don’t always need a new deisel engine if petrol has been used.. I did it this year with a VW Polo (work car). Drove nearly 10km down the motorway before it konked out. All it needed was a flush out and new pump and now runs fine. I think the damage was about $350. The humiliation at work was 10x worse.. haha.

    November 25th, 2013 at 9:29 am

  4. Chris says:

    Hi there
    Yes very good information and it should be publicised more for the general population there is no doubt
    The additional issue of concern lately is the “Diesel Blue” additive that seems to be available in a bowser format at some service stations.
    People apparently think it’s an “upmarket” brand of diesel and instead of adding a litre or two to a tank to improve performance the way in which it is apparently designed to be used, they often fill the tank with 50 plus litres of it, and it causes significant engine damage
    Be nice to have that kind of additive only available in bottles, rather than by the bowser when it is obviously not designed to used as a tank filling option in the first place.

    November 25th, 2013 at 9:31 am

  5. michael says:

    When I made the petrol into diesel car mistake, I was told by both the insurance (NRMA) and the Peugeot dealer service that there would be no short or long term consequences. I even drove the car some distance with about 75% petrol in my tank.

    That was two years ago and they appear to have been correct so far.

    November 25th, 2013 at 9:32 am

  6. ian says:

    Finally after 12 years of driving a diesel car, I did it……..a full tank of petrol. I didn’t realise. Drove it 1km……….car started to shake and conked out……….penny still not dropped. Restarted the car and reved…….conked out again…….can it get worse?? End result, despite what has been written in this article……….$570 later everything was fixed and fine. Car is a 2012 Mazda BT50.

    November 25th, 2013 at 9:56 am

  7. Mick says:

    A few years ago I managed to top up the tank on my Prado with a 20l jerry can of diesel. I realised fairly soon after and topped up the tank with unleaded and topped them up regularly to further dilute the diesel as the tank emptied.
    Except for it being a bit smokey over the first few days it doesn’t appear to have had any long term effects.

    November 25th, 2013 at 10:01 am

  8. john ceiley says:

    My neighbour filled his hilux with diesel. He had the ute towed home and we disconnected the fuel line and the return line under the bonnet We then pressurised the return line and this forced the fuel out of the tank without the need to get under the vehicle

    November 25th, 2013 at 10:40 am

  9. Trevor says:

    (b) getting one of the petrol pump attendants to do it for you.
    Which country do you find them in? Certainly not in Australia for about 20 years.

    November 25th, 2013 at 10:54 am

  10. Neil says:

    Given Caltex has not made it easy by using the word Vortex for both petrol and diesel, it would be great if Private Fleet and others like Drive and Consumer and Motoring organisations could draw Caltex’s attention to this problem and ask them to fix it. Caltex will take more notice if motoring organisations advise them of the problem rather than individual motorists.

    November 25th, 2013 at 10:56 am

  11. Steve says:

    AddBlue is used in large Diesel plant (semi trailers etc) to reduce Particulate emissions. They often have 50 Liter tanks which feed it into the engine through computer controlled injection.

    It would not be totally impractical for them to use bottles of it.

    November 25th, 2013 at 11:34 am

  12. Tangles says:

    3 times you idiot. how couldyou??
    Easy normaly use Shell servo. first two times were at Woolies discount and used to picking up from the same bowser I was in remote control and filled up.12Kms down the road putt putt black smoko. Pulled up checked the docket. DAM. RACV ride home on back of truck. Panic set in as my HiACE van is essential for work commitments.
    luckly found weekend mechanic who pumped out tank, flushed ,changed fuel filter, added cu metho and cup oil, 5 litres diesel then bled and started motor. all was good.
    the next 2 times I did my own repairs.
    Found the engin had reduced power and after some trouble shooting we replaced the cat. converter. We are now runnig better than new.
    Do not do it check twice before pulling the trigger.

    November 25th, 2013 at 11:38 am

  13. Rick says:

    you can get after market fuel cap for your diesel car , that pevents small petrol filler nozzle from being inserted.
    We got 2 different types for our diesels ,on line for about $70.00
    Cheap investment for not mixing up fuel
    One model i use is FuelSure ,
    The other fron RACQ Fuel Sure ,
    Hop this helps

    November 25th, 2013 at 11:39 am

  14. Glen says:

    Are you referring to ADDBLU. If so, this is for use in trucks and is not meant to go through the fuel system. It is injected into the exhaust to work in conjunction with a cat to remove NOx. Adding this (it’s basically liquid urea) to your fuel tank will kill your diesel engine pretty quickly.

    November 25th, 2013 at 12:46 pm

  15. Matt says:

    The one and only time I have ever pumped ULP into a diesel vehicle was at the truck bowsers at a large servo in Brisbane. There was diesel nozzle after diesel nozzle after diesel nozzle and one unleaded right in the middle, and I just happened to pick up the wrong one, pump a full tank then realize it didn’t smell right. When I went in to pay after realizing my folly, the attendant said I was the 4th person that day to make the same mistake!! No shit Sherlock!! One would almost think it was a conspiracy. Who would put a ULP nozzle amongst the diesel pumps at a truck stop?
    Fortunately my new Land Rover has a missfueling device in the filler neck that activates if it detects a skinny nozzle being inserted and blocks the filler neck.

    November 25th, 2013 at 4:15 pm

  16. GB says:

    BP has done the same thing as Caltex in naming Ultimate for both unleaded and diesel. I put a full tank of unleaded into my diesel ute thinking it was Ultimate diesel. Didn’t notice until the engine started to lose power about 15kms down the road, pulled over and checked my docket. Luckily had several empty drums on board, and following a call to my mechanic, proceeded to drain the tank and bleed the fuel system. It started first kick and hasn’t missed a beat since.

    November 26th, 2013 at 12:08 pm

  17. Robert says:

    Why don’t they just have diesel pumps with no petrol. Stilll can have the same lanes but one that is brightly coloured to make sure its diesel only

    November 27th, 2013 at 4:35 pm

  18. Chandana says:

    Many years ago, I added some diesel into my 1975 Ford Falcon FB S/W. The car was near new and after a rough run of about 30 km., I returned to the same servo and filled the tank with petrol. Thereafter, the engine ran with a minor flutter but performed quite well. Next weekend, I added a bottle of an engine cleaner through the carburettor for many hours, polluting the entire suburb with a layer of thick smoke. My engine performed perfectly thereafter.

    November 29th, 2013 at 9:43 am